Introduction to Lie Algebras and Representation Theory
《Introduction to Lie Algebras and Representation Theory》 · 印刷第 47 页 Section 10 的根系记号约定；第 53 页 Section 10.4「Irreducible root systems」，Lemma C 证明后的长短根定义段
Throughout this section $\Phi$ denotes a root system of rank $\ell$ in a euclidean space $E$, with Weyl group $\mathcal W$. […] In case $\Phi$ is irreducible, with two distinct root lengths, we speak of long and short roots. (If all roots are of equal length, it is conventional to call all of them long.)